The present invention is in the field of photochromic glass and is concerned with the treatment of photochromic glasses of the silver halide type to produce optically anisotropic glasses.
The basic disclosure relating to photochromic glasses is found in U.S. Pat. No. 3,208,860 to Armistead and Stookey. The glasses therein described may be briefly characterized as inorganic silicate glasses containing submicroscopic crystals of a silver halide, e.g., silver chloride, silver bromide or silver iodide. These crystals become darker in color when the glass is subjected to actinic radiation, such as ultraviolet light, but regain their original color when the actinic radiation is removed.
The behavior of silver halide photochromic glasses has been explained in terms of a photolytic reaction wherein the exposure of the glass-encased silver halide crystals to ultraviolet light causes the formation of light-absorbing minute silver particles in or on the silver halide phase. However, because the crystals are encased in a glassy matrix, the products of this reaction cannot diffuse away, and upon the removal of the activating light, the crystals return to the undarkened state.
Since the original inventon of silver halide photochromic glasses it has been verified that the photochromic behavior of silver halide crystals in glass is not closely limited by the composition of the host glass matrix in which the crystals are supported. Rather, a wide variety of silicate, borate and phosphate base glasses have been shown to be suitable for supporting a photochromic silver halide phase. And, while reversibly photochromic glasses comprising other photochromic phases such as copper-cadmium halides, hackamite, cerium, or europium have been developed, photochromic glasses of the silver halide type have become by far the most widely used of the photochromic glasses.
The fading of photochromic glasses from the darkened to the clear state in the absence of actinic radiation is explained in terms of the recombination of the silver metal produced by the photolytic decomposition with the halogen trapped at the silver halide crystal site by the matrix glass. The recombination is thought to occur by two independent processes. One involves a natural thermal recovery, accelerated by heating, which is referred to as thermal fading. The other occurs through exposure of the crystal site to light, typically though not always of longer wavelength (lower energy) than the actinic radiation which is used to darken the crystal, and is referred to as optical bleaching.
Factors affecting these two fading mechanisms are complex. In general, however, glasses which are photochromic because they contain silver halides can be optically bleached with varying degrees of efficiency depending upon the composition of the glass and upon the heat treatment used to precipitate the photochromic silver halide phase in the glass. Similarly, many of the silver halide photochromic glasses exhibit finite thermal fade rates.
A brief discussion of light polarization is also helpful in understanding our invention. In terms of the wave theory, light propagates with its characteristic electric E vector lying in a plane perpendicular to the direction of propagation. Linearly polarized light is light whose characteristic E vector is oriented in a fixed direction in this perpendicular plane. The state of polarization of the light is characterized by the relationship of this fixed directon to some reference direction, e.g., vertical or horizontal polarization with respect to the horizon, or perpendicular or parallel polarization with respect to a given axis in the plane.
A beam of natural light consists of all polarizations. That is, the direction of the E vector varies randomly in the plane perpendicular to the direction of propagation. At any instant the E vector may be resolved into components perpendicular and parallel to a selected reference direction in this plane. If one of these components is selectively absorbed as the beam of light passes through a medium, the light that is transmitted through the medium is considered to be linearly polarized. A medium having this property of selective absorption is called a polarizer.
The more common polarizers are composed of plastics, but glasses which linearly polarize light are also known. These glasses are also referred to as dichroic glasses, the term dichroic referring in this sense to the optical anisotropy of the glass with respect to its absorption coefficient. Thus light passing through such a glass experiences varying degrees of absorption, depending upon the direction of polarization of the light with respect to the glass. U.S. Pat. No. 2,319,816 to Land describes dichroic glasses produced by stretching glasses containing a minor metallic lead or gold phase. Polarizing glass comprising an elongated silver phase is described by Araujo and Stookey in Applied Optics, Volume 7, Number 5, pages 777-779 (1968).
Photochromic glasses which are non-polarizing in the clear state, but which polarize light in the darkened state, have been produced by stretching processes. Thus U.S. Pat. No. 3,540,793 to Araujo et al. describes stretched silicate glasses comprising parallel-oriented elongated crystals of silver halide which exhibit reversible photochromic and dichroic behavior. Also, Seward et al. describe, in U.S. Pat. No. 3,954,485, stretched copper-cadmium halide photochromic glasses exhibiting reversible polarizing properties.
Araujo et al. note that light polarized in a direction parallel to the direction of crystal elongation in their glasses is more strongly absorbed than light polarized in a direction perpendicular thereto, and suggest that the elongated crystals preferentially absorb light polarized in the direction of their alignment. Land indicates that anisotropic absorption in his glasses may occur over a wide range of light wavelengths, or may instead be limited to a rather narrow wavelength band, depending on the metal selected for incorporation in the stretched glass. Thus glasses which are strongly polarizing as to one wavelength band or color of light, but only weakly polarizing or non-polarizing with respect to other light wavelengths, have been produced.
Similar wavelength-dependent dichroism has been observed in silver chloride photographic emulsions which have been chemically or optically darkened and then bleached with polarized light. An early description of this phenomenon is provided by Cameron and Taylor in "Photophysical Changes in Silver-Silver Chloride Systems", Journal of the Optical Society of America, Volume 24, pages 316-330 (1934).